The present invention relates to a process for producing an aluminum support for a printing plate, and more particularly to a process for producing a printing plate support electrochemically--roughened so as to be suitable especially as an offset printing plate support.
Aluminum plates (including aluminum alloy plates) are employed as supports for printing plate especially as offset printing plate supports.
In general, an aluminum plate used as an offset printing plate member or support should have suitable adhesion and water retention properties as a photosensitive material.
In order to meet this requirement, the surface of the aluminum plate must be roughened in such a manner that it is uniformly and finely grained. The result of the surface roughening treatment will greatly affect the printing performance or service life of the printing plate member when it is used in an offset printing operation. Therefore, in the manufacture of a printing plate member it is critical to achieve the surface roughening treatment satisfactorily.
In general, an AC electrolytic etching method is employed to roughen the surface of a printing plate aluminum support. In the method, the current is an ordinary sinusoidal current, or a special alternating waveform current such as a square wave current. With AC current applied to a pair of electrodes which are, for instance, graphite, the surface of the aluminum plate is roughened. In general, the aluminum plate is only subjected to the surface roughening treatment once, and the depth of roughened surface therein are small over the entire surface, and accordingly the aluminum plate is insufficient to ensure good running characteristics. Therefore, a variety of methods have been proposed in the art to manufacture an aluminum plate suitable as support for a printing plate which is grained in such a manner that the depth of roughened surface are relatively large with respect to the diameter thereof and are distributed uniformly and closely.
The proposals made so far are based on optimizing parameters such as the ratio of electricity to be applied respectively to the anode and the cathode during electrolytic roughening with a-c current (Japanese Unexamined Patent Application (OPI) No. 65607/1979); the waveform of the voltage which is supplied from a power source (Japanese Unexamined Patent Application (OPI) No. 25381/1980); and combinations of variable currents to be applied per unit area (Japanese Unexamined Patent Application (OPI) No. 29699/1981).
In addition, the combination of a mechanical surface-roughening method and an electrochemical surface roughening method has been disclosed by Japanese patent Application Publication No. 16918/1982.
Despite such efforts, however, the depth of roughened surface produced by these prior art techniques are not sufficiently large and have such uneven depth profiles so as to provide a complexly undulating pattern. Not surprisingly, therefore, offset printing plates made by using aluminum supports having such defective pits are far from being satisfactory in terms of both printing performance and running characteristics.
Accordingly, an object of this invention is to provide aluminumsupport for a printing plate which is roughened in such a manner that the pits formed are greater in depth than in diameter and distributed uniformly and closely such that the plate has excellent printing performance and satisfactory running characteristic in an offset printing operation.